1. Field of the Invention
This invention relates generally to video monitor support circuitry and more particularly to interface circuitry between a video monitor and a digital computer.
2. Description of the Prior Art
Video monitors are commonly used output devices for small computer systems. The new breed of home computers almost exclusively use video monitors as their primary output device due to their relatively low cost and their high reliability factor.
A problem encountered with the less expensive types of video monitors (which are commonly used for home computer systems) is that their bandwidth response is so low that only a very limited number of characters can be displayed per line on their screen. For example, most home computers have video monitors that can only display 40 to 64 characters per line with readable resolution. This limitation of 40 or so characters greatly reduces the usefulness of the small computer, preventing its use for such applications as word processing, for example.
It would be extremely desirable to somehow modify an inexpensive video monitor so that more characters per line, perhaps 80, could be displayed with adequate and readable resolution. The most common of the inexpensive breed of video monitors have a bandwidth of about 8 MHz but must be driven at 14 MHz if they are to display 80 characters per line. Since horizontal lines on the monitor's screen must necessarily be composed of two or more side-by-side dots, their effective frequency is 7 MHz or less, and they are thus adequately displayed by the monitor. Since a vertical line is usually composed of a number of individual, vertically arranged dots each lasting only 1/f or 1/14 microseconds, in this case, the video monitor will not fully amplify their signal and thus will only weakly display the vertical line of dots on the screen. The effect of inputing a 14 MHz signal into an 8 MHz video monitor is then to produce characters having extremely bright horizontal components and weak, fuzzy vertical components.
A problem the prior art has apparently not addressed, then, is how to compensate for the resolution fall-off encountered when very high speed video signals are input into inexpensive video monitors.